Concrete and gypsum wallboard are typically characterized by the ability to withstand large compressive stress. However, tensile strength is generally poor. There has been a longstanding and unresolved need to improve the flexural strength of inorganic building materials. The use of metal, glass, wood, and polymer fibers as reinforcements to improve the tensile and flexural strengths has been attempted, but failed to solve some of the needed improvements.
For example, concrete is often reinforced by rebar, welded wire fabric or synthetic fibers to improve crack control. Rebar must be tied together when added to concrete to provide some structural integrity during earthquakes, for example. In addition, a number of building materials have been disclosed which have substantial quantities of cellulosic fillers, including wood particles and fibers. More expensive and brittle, glass fibers are used in place of wood in applications where greater fire resistance is required. Other fibers and fabrics, such as synthetics, have also been investigated, but failed to adhere to matrix materials used in construction materials. By fabrics, it is meant all woven or non-woven mats of at least one type of fiber, including fiberglass and textiles made of natural or synthetic materials.
Conventional reinforced, inorganic composites suffer from several disadvantages. For example, reinforcing wire mesh currently provides the most common crack control for concrete composites. However, wire is both expensive and prone to corrosion, which degrades the performance of the composite over time. Synthetic fiber reinforcements cost less and have high tensile strength, but have not been incorporated in a networked mesh and have poor adhesion with the matrix. Thus, results have shown comparatively poor crack control properties when compared to wire mesh.
Fiber reinforced gypsum composites also suffer from a variety of drawbacks. At low board weights, the majority of wallboard strength is provided by expensive multi-ply paper face sheets having oriented fibers. Particularly in high humidity climates, the paper facing of wallboard is subject to mold and mildew, which further deteriorates the mechanical properties of the board and produces disagreeable odors and may be harmful to human health. Also, paper face sheets are expensive, contributing approximately 40% to the overall cost of a wallboard.
In addition, conventional fibrous reinforcements, particularly glass, do not adhere well to the gypsum matrix, limiting the efficiency of adding fibers to the matrix. Glass fibers are also brittle and can become easily dislodged during handling, installation, or demolition, causing health hazards, such as skin irritation or lung damage.
The cementitious matrix in composites of concrete and wallboard should support a structural framework that resists compressive loading and transfers tensile loads to the fibers or fabrics that improve tensile strength of the composite material. However, strength and toughness of fiber-reinforced cementitious materials is strongly dependent on the interface between the fibers and the matrix. Most fibers have little interaction with a cement matrix, and thus poor interfacial adhesion. Fiber pullout occurs upon tensile loading. Incorporation of foam or lightweight filler into the matrix to reduce the density of the cementitious building materials exacerbates fiber pullout.
Sizing, an important step used in the preparation of fibers for weaving, involves coating fibers with a warp size. Warp sizes, or sizes, are film-forming polymeric materials which are applied to fibers to protect them from damage during the abrasive weaving process. These sizes are generally formulated to be removed after weaving. Sizing is similarly used in paper production to improve mechanical properties, dimensional stability, and resistance to water or solvents. However, as sizing is removed from fabrics, the use of sizing as a way to decrease pullout or increase interaction between fibers and a cementitious material has not been explored previously.